The use of optical fiber in the telecommunications industry is becoming the industry standard for data communications. It permits transmission over longer distances and at higher bandwidths than electrical wire cables. Typically, an optical fiber is obtained by producing a primary preform (or core rod), overcladding the primary preform and drawing it to form the optical fiber. For ease of handling and shipping, the optical fiber is then wound onto a spool at high speed (also referred as shipping or winding spool).
However, before winding the optical fiber on a shipping spool, the optical fibre is subjected to a testing phase tested to assess whether the fiber is suitable for cabling. One of the most important tests conducted on optical fibres is the proof test (also known as strength or tensile test). The purpose of the proof test is to ensure that the manufactured optical fibre can stand up to the tensile stresses which can occur while the fiber is being cabled or when the cable is being installed.
Thus, before winding the optical fiber on the spool, the optical fiber is passed through a proof-testing machine that applies a predetermined level of tensile stress to the fibre. If the optical fibre is too weak in terms of mechanical strength, it breaks under stress.
In the machine, the optical fibre is first guided at high speed, typically between 1500 and 3000 m·min−1, to an input pulling device and further to an output pulling device and then onto the shipping spool. The input and output pulling devices subject the optical fibre to a predefined value of tensile stress, as a result of which the fibre breaks if the fibre strength is insufficient. The machine also comprises several pulleys, which guide the fiber up to the spool and facilitate proper tension on the fiber as it is wound onto the spool.
During proof testing, the optical fiber is susceptible to break due to tensile stress applied by the two pulling devices on the fiber. When such a fiber break occurs, the broken fiber end tends to flail and whips around at high speed due to the high rotational speed of the spool. The uncontrolled broken end can impact the optical fiber already wound onto the spool and may cause irreversible damage to many layers of wound fiber (as the optical fiber is wound on the spool, the optical fiber is laid down onto the spool in successive layers). This phenomenon is commonly referred to as “whipping”. Fiber break during the proof test is unpredictable and, following such a break, the machine must be immediately stopped to prevent whipping damage to the wound fiber. However, because the break is unpredictable and the spool cannot be stopped instantaneously (basically because of its inertia), there is a period of time during which the spool will continue to rotate and the broken end can whip against the fiber already wound onto the spool, thus causing damage to the fiber.
Several known solutions were proposed in order to prevent fiber whipping.
The patent application WO 02/35210 discloses a proof-testing machine for optical fibre that ensures a continuous pulling and proof-testing process in case of a fiber break. To that end, the fibre end is guided in the case of break between the first and the second pulling device by means of a first channel, which guides the fibre to the second pulling device. After achieving the second pulling device, the fibre end is guided into a second channel which is off the normal fibre track and along which the fibre is guided into a scrap processing system. The fibre is then guided from the second channel to the normal track, along which the fibre is guided onto a winding spool. This kind of implementation is however complex to implement. In addition, it does not enable that all optical fiber is accumulated on the spool after detecting a break, without damaging the fiber already spooled on the spool.
The patent application WO 99/55612 discloses a fiber proof-testing machine for reducing or eliminating fiber whipping phenomenon. The machine comprises a whip shield surrounding the winding spool and a fiber entry whip reducer positioned between the output pulling device and the winding spool. The entry whip reducer includes pulleys and a guide channel configured such that the broken end is maintained against the guide channel by centrifugal force imparted onto the fiber by the curvature of the channel and forward motion of the fiber produced by the spool, thereby producing a trajectory such that the loose end is maintained against the whip shield. By maintaining the broken end of the fiber against the guide channel during fiber entry, whip damage can be reduced or eliminated. In practice, however, the rotational speed of the spool is also relatively high, and so the broken end can flail on several revolutions of the spool, thereby increasing the risk that the broken end finally impacts the fiber already wound onto the spool. In other words, this solution does not ensure eliminating completely whipping phenomena. In addition, adding a fiber whip reducer adding complexity to the machine. By the way, it is complex to clean the machine and to maintain the protection shield alignment.
The patent application WO 01/46055 proposes a fiber broken end cutting apparatus arranged to separate the broken end from a wound segment to reduce fiber whip damage. An active cutting element is positioned to cut off a segment of the broken end from the optical fiber being wound onto the rotating spool, which otherwise might be caused to whip into the fiber already wound on the spool. However, a drawback of this technical solution is that it requires the addition of an active fiber cutting system relatively complex to implement in a proof-testing machine. Finally, this solution does not offer the possibility of recovering substantially all of the optical fiber without whipping.